Even if the light goes off it still stores the code. If you have never cleard any of the codes I would just go ahead and do that first. Then drive it a bit and see if any come back.good find! Im going to have to do this to my 1991 mk2. Car runs great but sometimes when i drive the car at highway speeds or over 3 to 4 k rpm my check engine light comes on and then goes away. Does anyone know why this happens?